After releasing an Elastic IP address, it is released to the IP
address pool and might be unavailable to you. Be sure to update your
DNS records and any servers or devices that communicate with the
address. If you attempt to release an Elastic IP address that you
already released, you'll get an AuthFailure error if the
address is already allocated to another AWS account.

[EC2-Classic, default VPC] Releasing an Elastic IP address
automatically disassociates it from any instance that it's associated
with. To disassociate an Elastic IP address without releasing it, use
DisassociateAddress.

[Nondefault VPC] You must use DisassociateAddress to disassociate the
Elastic IP address before you try to release it. Otherwise, Amazon EC2
returns an error (
InvalidIPAddress.InUse ).